This invention relates generally to expandable polystyrene beads and, more particularly, to an anti-lumping agent useful for the formation of expandable polystyrene beads and low density foams formed therefrom.
Methods for the production of expandable polystyrene beads are well known. Such expandable beads generally comprise a mixture of an expandable styrene or styrene derivative polymer and a blowing agent. The expandable polystyrene beads are generally initially formed into beads of relatively small size having a diameter of from about 0.2 to 4 millimeters. The beads are generally formed in a suspension polymerization reaction. The blowing agent is introduced into the beads either during or after the polymerization reaction. The blowing agent is generally homogeneously dispersed within the polymer and the blowing agent may be, in general, a hydrocarbon which is gaseous or liquid under normal conditions and which does not dissolve the polymer. In addition, the boiling point of the blowing agent must be below that of the softening point of the polymer.
Typically, the expandable polystyrene beads go through at least a two-step process prior to molding. In the first step, the expandable polystyrene beads are expanded to form what are known as xe2x80x9cprillsxe2x80x9d or as a xe2x80x9cpre-puffxe2x80x9d. This step comprises heating the beads to a temperature above their softening temperature and thereby above the boiling point of the blowing agent resulting in vaporization of the blowing agent and expansion of the beads to form individual particles of foam prills. These prills are quite fragile and prior to molding in the second step it is generally necessary to allow the prills to age for a period of time. To age the prills they are maintained under a normal atmosphere. During the aging process blowing agent within the prills diffuses out and air from outside diffuses into the prills. Once the prills have aged they can be placed in a mold and heated again such that the prills expand to fill the molding space and form a molded object. During this period the prills expand and fuse or adhere to each other to form the molded object. The molded objects can either be large blocks which are subsequently cut with a hot wire cutter into sheets or the prills can be molded directly into a particular shape.
Increasingly manufacturers of molded expanded polystyrene are attempting to shorten the expansion time frame and to mold increasingly lower density foams, thereby leading to an increase in profitability. One negative result of expanding foams to lower densities is that as the density of the foam decreases there is a significant increase in xe2x80x9clumpingxe2x80x9d of the prills. This problem is especially severe for expandable polystyrene beads that include low levels of blowing agents. Lumping is defined as the expandable polystyrene beads fusing together during the initial expansion step, which results in formation of large lumps of prills. These large lumps create at least two problems. First, the expansion process becomes difficult to control because of the formation of these large lumps. The second problem is that if the lumps are sufficiently large enough the material becomes unusable for the subsequent molding steps. In an attempt to control this lumping phenomenon, a number of external coatings have been utilized. Some examples of commonly used external coatings include zinc stearate, glycerol mono stearate, ethylene-bis stearamide (acrawax), and silica. Unfortunately, these commonly used lubricants are not successful in preventing lumping when the expandable polystyrene beads are expanded to very low densities.
Thus, it would be very advantageous to provide an anti-lumping compound that could be utilized with expandable polysytrene beads containing low levels of a blowing agent that would permit these beads to be expanded to very low densities previously not routinely obtainable.
In general terms, this invention provides expandable polystyrene beads having a reduced lumping tendency. In particular, this invention provides a class of anti-lumping agents for use with expandable polystyrene beads which significantly reduces lumping when expanding the polystyrene beads into low-density foams of from about 1.5 to 0.75 pounds per cubic foot (pcf), and more preferably from about 1.2 to 0.85 pcf. This invention also finds special utilization when preparing expandable polystyrene beads having low levels of blowing agents.
In a first embodiment, the present invention is an expandable polystyrene bead comprising an expandable styrene or styrene derivative polymer and from about 2 to about 5% by weight, based on the total weight of the bead, of a C3 to C6 hydrocarbon blowing agent, wherein the bead is coated with an anti-lumping agent comprising a block copolymer of propylene oxide and ethylene oxide blocks, said block copolymer comprising from about 80% to 65% ethylene oxide and from about 20% to 35% propylene oxide, and having a hydrophilic-lipophilic balance (HLB) value of from about 16 to 29.
In a second embodiment, the present invention is a method for the formation of an expandable polystyrene bead comprising the steps of: forming an expandable polystyrene bead containing from about 2% to about 5% by weight, based on the total weight of the bead, of a C3 to C6 hydrocarbon blowing agent; and coating the expandable polystyrene bead with an anti-lumping agent comprising a block copolymer comprising blocks of ethylene oxide and propylene oxide, wherein the block copolymer comprises from about 80% to 65% ethylene oxide and from about 20% to 35% propylene oxide, and has an HLB value of from about 16 to 29.
These and other features and advantages of this invention will become more apparent to those skilled in the art from the detailed description of a preferred embodiment.
In general terms, this invention provides expandable polystyrene beads having a reduced lumping tendency. In particular, this invention provides a class of anti-lumping agents for use with expandable polystyrene beads that significantly reduces lumping when expanding the polystyrene beads into low density foams of from about 1.5 to 0.75 pcf. This invention also finds special utilization when preparing expandable polymer beads having low levels of blowing agents.
The expandable polystyrene beads are generally formed via the well known aqueous suspension method wherein the monomers are polymerized and combined with a blowing agent and other additives to form the beads. The beads are then separated from the aqueous portion of the suspension, washed and dried. In at least a first expansion step the beads are heated, usually via steam, to a temperature above the boiling point of the blowing agent. This heating causes vaporization of the blowing agent and expansion of the beads into a form known as a prill or prepuff. The prepuff is then allowed to age for a sufficient period of time, usually suspended in mesh bags. During the aging process external air equilibrates across the prepuff and some residual blowing agent leaves the prepuffs. The prepuff is then placed into a closed mold and heated again to form a shaped object, be it a block or a more complex shape. In some procedures the prepuffs are expanded 2 to 4 times prior to their use in the closed mold.
In the aqueous suspension polymerization reaction the monomers used to form the polymer comprise styrene or styrene derivatives. The styrene derivatives include: alpha-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, ar-ethylstyrene, ar-vinylxylene, ar-chlorostyrene, and ar-bromostyrene. The derivatives may include minor amounts of divinylbenzene, methylmethacrylate, or acrylonitrile. The preferred monomer is styrene. The monomer or mixture of monomers is suspended in an aqueous solution and polymerized. The blowing agent is generally added during the suspension polymerization, but can be added during later processing steps. The suspension polymerization also generally includes: chain transfer agents, suspension stabilizers, and polymerization catalysts. Typical chain transfer agents include: dimeric xcex1-methylstyrene. After polymerization, the polymer is generally present in an amount of from about 93 to 98 weight percent based on the weight of the bead.
Blowing agents suitable for the present invention generally comprise C3-C6 hydrocarbons and mixtures thereof, with pentane isomers being preferred. The blowing agent must have a boiling point below that of the melting point of the polymer so that upon heating of the expandable polystyrene beads the blowing agent will vaporize and leave the beads thereby expanding the bead. Generally a boiling point below 70xc2x0 C. is preferred. Suitable blowing agents include, for example: propane; butane; isobutane; n-pentane; isopentane; neopentane; cyclopentane; methylcyclopentane; 2-methyl pentane; 3-methyl pentane; 2,2-dimethylbutane; 2,3-dimethylbutane; pentane petroleum distillate fractions; hexane; cyclohexane; methylcyclohexane; and hexane isomers. Blowing agents are generally used at levels of from about 2 to about 5 weight percent, more preferably from about 2.5 to about 4 weight percent based on the total weight of the bead.
As discussed above, the suspension polymerization usually is carried out in the presence of suspension stabilizers and polymerization catalysts. Common suspension stabilizers include: molecular colloids, such as, polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP); the pickering salts such as Ca3(PO4)2 in combination with an extender such as dodecylbenzol sulfate. Typical polymerization catalysts include radical catalysts such as dibenzoylperoxide, tert-butyl perbenzoate, and dicumyl peroxide. Other suitable catalysts are well known to one of ordinary skill in the art.
The expandable polystyrene beads can also include other additives like flame retardants based on organic bromo or chloro compounds, such as, tris(dibromopropyl) phosphate, hexabromocyclododecane, chloroparaffin. Other additives include antistatic agents, stabilizers, dyes, lubricants, and fillers. To aid in demolding the object formed from the expandable polystyrene beads one may also include compounds like glycerine ester, hydroxycarboxylic acid ester.
As discussed above, typical anti-lumping agents include zinc stearate, melamine formaldehyde condensates of silica gel, glycerol mono stearate, ethylene-bis stearamide (acrawax), and silica. The typical agents are not effective when expanding beads to lower densities or when using the indicated low levels of blowing agents. The present invention discloses a new class of anti-lumping agents that are block copolymers.
By the term block copolymers it is meant a polymer having blocks formed of a plurality of a repeating unit, such as ethylene oxide (EO), that are attached to additional blocks of a plurality of another repeating unit, such as, propylene oxide (PO). A given anti-lumping copolymer may comprise a series of these blocks in an arrangement such as: EOx-POy; EOx-POy-EOz; POx-EOy-POz; etc. The particular arrangement of the blocks along with the overall HLB value for the polymer are important characteristics. It is preferable to match the arrangement of the blocks to the overall hydrophobicity of the expandable bead. In other words, when the bead is more hydrophilic, then an arrangement of EOx-POy-EOz is beneficial. When the bead is more lipophilic, then an arrangement of POx-EOy-POz is more beneficial. In general, it is preferable that the amount of EO, which is generally hydrophilic, in the block copolymer range between about 65 and 80% by weight based on the total weight of the copolymer. The amount of PO, which is generally lipophilic, is preferably from about 20 to 35% based on the total weight of the copolymer. It is preferable that the number average molecular weight of the copolymer be from approximately 4,000 to 15,000, preferably from about 6,000 to 11,000, most preferably from about 7,000 to 10,000. In general, the viscosity of these copolymers is from about 450 to 2,500 cps at 77xc2x0 C., provided they are in a solid form at 20xc2x0 C. The melt point of the copolymers ranges from approximately 45 to 60xc2x0 C.
In general, it is preferable that the HLB value of the copolymer range from about 16 to 29, depending on the surface characteristics of the bead as discussed above, when it is utilized to coat an expandable polystyrene bead prepared by an aqueous suspension reaction. The HLB value is a semi-empirical number determined from the weight percentage of hydrophilic groups to lipophilic groups within a molecule or mixture. The value is a general indication of the polarity of the molecules and ranges from 1 to 40. The HLB value increases with increasing hydrophilicity. Preferred HLB values of the block copolymers are from about 19-29, and most preferably from about 24 to 29.
Methods for formation of the block copolymers described above are well known in the art and will only briefly be described. Typically, one utilizes an initiator molecule such as a lower molecular polyhydroxyl or polyamine initiator molecule. By way of example, the initiator can be a diol, triol, diamine or triamine. Examples include propylene glycol, ethylene glycol, glycerol, and ethylenediamine. The initiator is utilized in an alkali-catalyzed polymerization reaction with alkylene oxide compounds such as ethylene oxide or propylene oxide to generate a polyetherol. When one carries out the alkali-catalyzed polymerization sequentially using only a single alkylene oxide at a time, one can form block copolymer polyetherols that are useful as the anti-lumping agents of the present invention. One can also utilize amines such as ethylenediamine as the initiator in these reactions. Suitable block copolymers include the Pluronic(copyright) or Pluronic R(copyright) copolymer series produced by BASF Corporation. Preferred block copolymers include the PLURONIC F68 and F108 products.
After formation of the polystyrene beads they are separated from the aqueous phase, washed and dried. One method of drying the beads is to pass them into a screw conveyor that feeds a heated airveyor for flash drying. One method for application of the new anti-lumping agents is to add it to the beads in the screw conveyor. Some mixing occurs in the screw conveyor and the rest occurs in the flash dryer. The block copolymer anti-lumping agent can be added alone or with other anti-lumping agents. The anti-lumping agent is generally incorporated into the polymer bead in an amount of from about 250 to 2500 ppm, based on the weight of the polymer. Preferably, the anti-lumping agent is present in an amount of from about 300 to 1250 ppm.
Once dried the beads are sized into fractions. The formed beads generally have a diameter of from about 0.2 to 4.0 mm. Another method for adding the new block copolymer anti-lumping agents to the beads comprises adding the agent to a powder blending system after sizing of the beads. These blending systems typically include a plow blade mixer such as the Littleford FKM mixers. The anti-lumping agent and other additives are combined with the beads and mixed using a plow blade mixer.
The initial expansion of the beads is generally carried out in a closed vessel batch expander with a steam injection process. Typical examples of such expanders include: Weiser VN400, Kurtz KV1000, and Dingledein VA2000. The beads are passed through the expander and are heated such that the blowing agent vaporizes thereby expanding the beads. The flow rate of the beads through the expander determines the amount of expansion and it is generally reported as pounds per hour per cubic foot of expander volume, or for a given expander as pounds per hour. During the initial expansion stage lumping of the expanding polystyrene beads can occur. To achieve low density polystyrene beads the expansion time must be increased, but in the past increasing the expansion time has also increased the amount of lumping. With the anti-lumping agent of the present invention it is possible to achieve low density polystyrene beads without an increase in the amount of lumping. As explained above, when the beads lump, they can be difficult to use in a subsequent molding process.